There are no parathyroid glands in fishes and some aquatic urodeles. Instead, the pituitary gland contains a hypercalcemic hormone (probably prolactin). All other vertebrates have functional hypercalcemic parathyroid glands. In all submammalian vertebrates, estrogen injections elicit an increase in plasma calcium levels. Prolactin, estrogen and parathyroid hormone have been shown to stimulate renal 25-hydroxyvitamin D3-1-hydroxylase in birds. This enzyme converts 25-hydroxyvitamin D3 to 1,25-dihydroxyvitamin D3, the biologically active metabolite. The relationships among these three hypercalcemic hormones in renal enzyme stimulation have not been investigated because of the difficulties in performing simulataneous hypophysectomy and parathyroidectomy in birds. We propose poikilotherm animal models which will enable us to study these interrelationships. Hypophysectomy and parathyroidectomy can be performed easily in these animals. The evolution of the renal vitamin D metabolizing enzyme system will also be studied. We will use autoradiography to localize the renal enzymes in perfused bullfrog kidneys. In situ bullforg renal perfusion was perfected and is used extensively in the principal investigator's laboratory.